1. Field of the Invention
The present invention is related to a method of depositing Zinc Oxide (ZnO) using a multistep process, for example on a semiconductor material such as a group III-nitride based optoelectronic device.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Zinc oxide (ZnO) is a group II-VI direct band gap compound semiconductor material used in a wide array of electronic applications. Given its wide band gap and ease of doping, ZnO films can simultaneously achieve high optical transparency and low electrical resistivity. Transparent electrodes composed of ZnO can be used for optoelectronic devices such as light emitting diodes, laser diodes, photovoltaics, and thin film transistors. In addition to those devices, conductive films containing zinc oxide have been shown applicable in varistors, piezoelectric transducers, as well as gas, chemical and biological sensors. ZnO films can be readily made highly conductive through doping with substitutional atoms such as group three elements (i.e. Al, Ga, and In) [1]. Compared to conductive films such as indium tin oxide (ITO), ZnO can typically be deposited at lower costs using a variety of physical and chemical deposition techniques. ZnO films can also be easily patterned using either wet or dry etching methods. However, using a single deposition method to produce high quality ZnO films for device applications can be challenging.
ZnO on GaN
One application for which conductive ZnO films are especially well suited is as a transparent electrode for III-N based optoelectronics [2]. Given that ZnO and III-N materials share the same wurtzite crystal structure and are closely latticed matched to one another, epitaxial films of ZnO can be deposited on this material system. This allows for the growth of ordered crystalline ZnO films with high carrier mobilities and low optical absorbance, due to the reduction of phonon/electron scattering centers such as grain boundaries, point defects, and lattice distortions. In addition, the ZnO high refractive index of ˜2 allows for more efficient light extraction from III-N light emitters such as GaN, whose refractive index is ˜2.5, by increasing the critical angle for total internal reflection. Moreover, ZnO's ease of patternability, whether through selective area growth, patterned etching, or roughening etches, further increases light extraction by reducing the probability of photons meeting the criteria for total internal reflection.